1. Field of the Invention
This invention relates generally to the field of phase locked loop frequency synthesizers. More particularly, this invention relates to wide band phase-locked loop frequency synthesizers for angular modulation systems requiring constant modulation levels over the entire radio frequency band of operation of the synthesizer.
2. Background of the Invention
It is desirable and sometimes necessary due to government regulations to produce a constant level of maximum modulation from a transmitter at each frequency at which the transmitter is operated. This requirement creates a particularly difficult problem to overcome in the field of wide-band frequency synthesized transmitters.
Conventional voltage controlled oscillators, which are used in the phase locked loop frequency synthesizers for such broadband transmitters, typically utilize voltage variable capacitor elements known as varactor diodes. These varactor diodes can resonate with an inductive element such that as the voltage applied to a varactor diode varies, the capacitance changes resulting in a shift in the resonant frequency of the oscillator. The output frequency versus control voltage characteristics of such an oscillator is shown in a broken line as curve 10 of FIG. 1. The VCO gain factor K.sub.0 (expressed in MHz/volt) exhibits a similar non-linear shape as shown in FIG. 2 as curve 15, also shown in a broken line.
In a frequency modulation system, the control voltage of the VCO is typically modulated with audio frequency voltages to produce a resultant FM deviation. If the conventional voltage controlled oscillator is operated over a wide frequency band the non-linearities of FIGS. 1 and 2 manifest themselves as a variation in FM deviation across the frequency band.
Referring to FIG. 3, this variation in FM deviation or non-linearity is seen in curve 20 shown in a broken line. For a constant magnitude audio input, modulation is directly proportioned to K.sub.0 so that at lower frequencies where a low level of control voltage is utilized, resultant modulation may be quite high, while at higher frequencies where the control voltage must be at a higher level the resultant modulation falls off substantially. Such wide variations and resultant modulations can produce undermodulation at higher frequencies, overmodulation at lower frequencies and thereby produce a failure to meet government regulations.
Various compensation techniques have been utilized to effect a flattening of the resultant modulation vs. control voltage curve such as shown in FIG. 3 as solid line curve 120 so that for example, FM deviation remains constant over the entire synthesizer frequency range. One such technique utilizes the forward bias characteristics of a junction diode in an attempt to counteract the non-linearities of curve 20. Since these non-linearities are not a perfect match, the compensation is only good for a narrow range of frequencies compared with the present invention.
Other techniques have involved attempting to linearize and flatten the characteristics of the varactor diode itself in order to obtain flat modulation over a particular frequency range. These attempts generally result in highly complex and expensive circuit arrangements.
Such techniques have met with limited success while requiring a large number of circuit elements. These techniques have provided an acceptable response over a frequency range perhaps as great as several megahertz with acceptable temperature performance and frequency stability. However, for wideband synthesizers requiring linear modulation over a frequency range having a magnitude of 10 to 20% of the RF center frequency, these compensation techniques are grossly inadequate.